Since the method of preparing a monoclonal antibody was established, as a method for assaying a biologically-relevant substance of interest in a specimen, an immunoassay such as enzyme immunoassay has been employed as the main assay method. When using such an immunoassay, it is possible to perform direct assay because of its high specificity, and assay can be performed with high sensitivity. Further, recently, regarding these assay methods, steps from the step after setting a collected specimen to the step of obtaining assay results have been automated by the assay system. Further, in order to accelerate assay more, reagents in which concentrations of a solid-phased antibody and a conjugate are higher than ever before have been developed. However, when using reagents in more concentrated form, nonspecific reactions, which are conventionally unrelated at the time of assay, are often caused.
It is considered that causes of nonspecific reactions are the variety of target substances, the presence of immune analogs, the variety of antigens and the variety of antibodies. In the immunoassay system, as substances causing various nonspecific reactions, IgA-type, IgM-type and IgG-type heterophilic antibodies (antibodies that react between animals of different species: HAMA, anti-BSA antibody, etc.) and biological components (e.g., rheumatoid factor, cryoglobulin and M protein) are present, and there is a case where they show false positive in immunoassay as a result of a nonspecific reaction (see Non-patent documents 1, 2 and 3). Moreover, since cancer-associated antigens such as sugar chain exist on the surfaces of bacteria, in the case of infection caused by bacteria, false positive is often shown in cancer tests without cancer. Furthermore, there is a case where false positive is shown due to a nonspecific reaction caused, for example, by bacterial infection at the time of surgery for removing an organ from a cancer patient.
Further, a nonspecific reaction often occurs during pregnancy or when being affected with liver disease, kidney disease or the like. Moreover, recombinant antigens are used for many recently-developed reagents. It is known that due to the presence of bacterial components used at the time of preparation of these recombinant antigens, antibodies against these bacteria affect the assay system. In general, when performing immunoassay, inhibitors against these nonspecific reactive substances are added. However, since there is a limitation on the adding amounts for reagents, sufficient inhibitory effects are not necessarily obtained. Therefore, it is difficult for currently-used assay systems to remove nonspecific reaction factors and to sufficiently inhibit nonspecific reactions to perform assay.